双重检查模式实现单例模式，使用volatile，代码如下：

public class Singleton {
	private static volatile Singleton instance;
 
	private Singleton() {
	}
 
	public static Singleton getInstance() {
		if (instance == null) {
			synchronized (Singleton.class) {
				if (instance == null) {
					instance = new Singleton();
				}
			}
		}
		return instance;
	}
}

参考

https://www.iteye.com/topic/652440
https://blog.csdn.net/sinat_15891495/article/details/78209351
https://blog.csdn.net/lin_coffee/article/details/79890361
https://blog.csdn.net/zhangzeyuaaa/article/details/42673245